The present invention relates to cellular systems for transmission of information by radio between an infrastructure and moving bodies constrained to move along a determined path, the infrastructure having fixed transmitter/receiver stations distributed along the path.
A major application of the invention lies in the field of railways, in particular rail networks which are, at least in part, underground, where multiple reflections are prone to occur. Nevertheless, the invention is also usable in other fields, and in particular for communication with aircraft.
In rail applications, proposals have already been made to use radio transmission to replace links via low frequency carrier currents in the rails or in cables. In particular, proposals have been made to use the GSM standard for providing such transmission. Radio links have the advantage of not requiring special cables, or special dispositions for using rails that are already in use for traction current and in certain signalling installations. However, radio links need dedicated frequencies, are subject to unfavorable propagation conditions in tunnels which lead to multiple reflections, and may be disturbed by radio interference.
Conventional systems based on the GSM standard, which is a time division multiple access (TDMA) system, making use of base stations that are generally placed in the middles of cells do not make it possible to avoid all of those problems, even when implementing slow frequency hopping (SFH), unless radiating cables are used, which are expensive.